1. Field of the Invention
The present invention relates to a method of controlling a frequency converter of a switched reluctance machine, which includes current regulation with hysteresis, with which the current switch is clocked with a frequency, which depends on the width of a hysteresis band.
2. Prior Art
Different methods for controlling a frequency converter for a switched reluctance motor are described in Chapter 4, Dynamic Operation, on pp. 53 to 61 of the book xe2x80x9cSwitched Reluctance Motors and Their Controlxe2x80x9d by T. J. E. Miller, MAGNA PHYSICS PUBLISHING AND CLARENDON PRESS, OXFORD, 1993. A frequency converter circuit, which provides a switch for the current supply, a commutator switch and two free-running diodes associated with these switches for each phase of the machine, is usually used. The switches usually comprise transistors. One such switch is shown, e.g. in FIG. 4.1 on page 53 of the above-mentioned reference. These switches can be operated in both a hard and also soft interruption mode. In the hard interruption mode both the current switch and the commutator switch of a phase are clocked on and off simultaneously during its current conducting phase. In the soft interruption mode only the current switch is clocked on and off during the current conducting phase while the commutator switch of this phase is turned on during the entire current conducting phase. This soft interruption mode (soft chopping) is preferred for motor operation of a switched reluctance machine, because it causes less noise and electromagnetic interference. The current is thus controlled during the hysteresis-current control described on page 62 of the above-mentioned reference so that it should remain at a given value in a stage between a maximum and a minimum value, at which the provided current switch is clocked on and off accordingly. The clock frequency, with which the supplied current is switched on and off, decreases with increasing inductivity and increasing flux interlinking in the associated phase. The clock frequency is also variable according to the predetermined hysteresis band. This leads to unpleasant and in some cases harmful noise problems.
EP 0 848 491 A2 describes a process and circuit arrangement for operating a switched reluctance motor, in which a hysteresis-current regulation with adjustable hysteresis band is provided. The current switch is clocked. The clocking is kept constant and the width of the hysteresis band is predetermined and changed so that the clock frequency is maintained constant during each occurring current load and current demand. Also the clock frequency is monitored and a frequency error signal is generated, which acts to change the width of the hysteresis band, so that the clock frequency is maintained substantially constant. Noise problems can be largely reduced by this system, since no low frequencies occur. However the clock frequency must generally be measured, which is difficult in practice and is expensive. Furthermore this system is constructed as a true regulating system with the frequency as a feedback signal. However because of the constant clock frequency noise problems occasionally occur.
It is an object of the present invention to provide a simpler and more economical method of controlling a frequency converter circuit of a switched reluctance machine, which also operates according to another principle than the current prior art methods.
It is another object of the present invention to provide a simple and economical method of controlling a frequency converter circuit of a switched reluctance machine, which avoids disadvantageous noise problems.
These objects and others, which will be made more apparent hereinafter, are attained in a method of controlling a frequency converter of a switched reluctance machine, which includes current regulation with hysteresis, and in which a power switch is clocked with a frequency, which depends on the width of the hysteresis band.
According to the invention the width of the hysteresis band is varied according to the rotational speed of the reluctance machine, particularly so that a difference between maximum and minimum threshold values with which the current for the reluctance machine is switched on and off either increases with decreasing rotational speed, decreases with increasing load, or both.
The method of controlling a frequency converter of a switched reluctance machine according to the invention has the advantage that a substantially simpler indirect control of the width of the hysteresis band is provided. The width of the hysteresis band depends on the following parameters: rotation speed, phase current and rotor position. These parameters are necessary for regulating the drive and are thus already present. Moreover the essential advantage of the method is that no additional signal is required and thus the required work and expenses are reduced.
Additional improvements and advantages are provided by the features of preferred embodiments claimed in the appended dependent claims and described in the following description.
In a particularly advantageous embodiment of the method the width of the hysteresis band is varied linearly with rotational speed, load and/or current of the reluctance motor, especially so that the clocking frequency with which the power switch is clocked is maintained substantially constant.
According to a very advantageous preferred embodiment of the method according to the invention for avoiding noise-critical resonance frequencies the method includes making a targeted change in the width of the hysteresis band in order to change the clocking frequency when the rotational speed of the reluctance machine is at a speed value at which the clocking frequency which is substantially constant is in the vicinity of a noise-critical resonance frequency. In this way the noise-critical resonance frequencies may be avoided in an elegant manner, since the targeted change of the clocking frequency is conducted with a suitable change of the width of the hysteresis.
A special embodiment of the method of the invention includes changing the clocking frequency from a higher or lower constant value to a lower or higher constant value during the targeted change so that the width of the hysteresis band is increased or decreased respectively.
In a particularly preferred embodiment of the method the reluctance machine is operated as a reluctance motor and the power switch comprises transistors.
According to an additional advantageous embodiment of the method of the invention the width of the entire hysteresis band is lowered or raised while maintaining its width by a predetermined amount.
According to an additional embodiment of the method of the invention the width of the hysteresis band is changed, especially periodically, according to a relative position of a rotor of the reluctance machine within a revolution of the rotor.
Before describing the preferred embodiment in the drawing, an introduction will be given first regarding the general control of switched reluctance machines. These machines are usually voltage or current regulated. In voltage regulation the reduction of the average voltage over the phase occurs by clocking transistors, which are used as power switches. This clocking occurs with a fixed frequency and variable on/off ratio.
In current regulation the phase current is controlled between two fixed values. The transistor used as the power switch is switched off when the upper threshold is exceeded and switched on which the current falls below the lower threshold. In this way the phase current can be made constant within a hysteresis band, whose bandwidth is given by the difference between both thresholds. The clocking frequency of the power transistors adjusts itself variably according to the speed, the rotor position and the width of the hysteresis band as well as the absolute height of the threshold values. Up to the present usually the width of the hysteresis band is kept constant. However in the method described in EP 0 848 491 mentioned above the clocking frequency is kept constant, while the width of the hysteresis band changes accordingly.
The problem is that the components and the circuit losses limit the maximum possible clocking frequency. Because of this fact the width of the hysteresis band for the current regulation is adjusted so that the maximum clocking frequency is not exceeded for slower motor speeds, when the reluctance machine is operated as a motor. When the width of the hysteresis band is kept constant over the entire speed range, then the clocking frequency for higher speeds strongly decreases. Because of that it is even possible that the clocking frequency drops until it is in an audible range. The variable clocking frequency can also reach the range of the noise-critical resonance frequencies, so that the noise behavior of the reluctance machine is clearly degraded.
Some help is provided by the method according to the invention since the width of the hysteresis band or bands is changed according to the speed of the reluctance machine. This change takes place so that the spacing between the maximum and minimum threshold values, with which the current is switched on and off, increases with decreasing speed and decreases with increasing load. The width of the hysteresis and/or hysteresis band is determined by the spacing between the maximum and minimum threshold value, with which the current is switched on and off.
According to the invention at a first starting point the width of the hysteresis can be changed linearly with the speed. Instead of that the speed-dependent change can also take place according to the load and/or current. This advantageously happens so that the clock frequency, with which the power switch is clocked, is substantially constant. If this constant clock frequency is in the range of certain noise-critical resonance frequencies in certain applications, such as at a predetermined speed of the controlled reluctance machine, then a special embodiment of the method according to the invention provides a targeted change of the width of the hysteresis band in order to obtain a desired change in the clock frequency. The clock frequency is preferably reduced from a higher constant to a lower constant value, whereby the width of the hysteresis band is increased. Also a further increase is conceivable, as the desired change occurs in the reverse way.
The switched reluctance machine is preferably operated as a reluctance motor and is provided with transistors as power switches. It is also possible to raise or lower the entire hysteresis band a predetermined amount while maintaining its width. Since the inductance changes periodically with rotor position in the switched reluctance machine, the width of the hysteresis band can be changed according to the rotor position in one revolution to maintain the clock frequency constant and of course periodically.
A block diagram of the control according to the method of the invention for the example of rotation speed regulation is shown in FIG. 1. Fundamentally also position and torque regulation are also possible.